1. Field of the Invention
The present invention relates to an optical scanner which has the function of reading images and which may be employed in, e.g., a facsimile machine or a copying machine.
2. Related Art Statement
There is known a laser facsimile machine or a copying machine which has a first optical system for reading images and a second optical system for recording images, the two optical systems being independent of each other. In particular, as the image-reading optical system, there has been known an optical scanner which, as described below, scans an original having an original image thereon and reads the original image using a one-dimensional-array optical sensor (i.e., "line sensor"). A line sensor employed in an optical scanner is disclosed in Japanese Patent Application laid open for inspection under Publication No. 55(1980)-159412.
FIG. 12 shows a conventional copying machine. The copying machine has an image-reading optical system including a light source 113 which emits a linear light toward an original (not shown) placed on a support surface of a transparent original-supporting member 114. The optical path of the linear light reflected from the original is deflected by a full-speed scan mirror 111 and then by a half-speed scan mirror 112. The linear light reflected by the half-speed scan mirror 112 is converged by an image-forming lens 116 as a light converger, so that the converged light is incident to an array of CCDs (charge-coupled devices) 115 which extends in a direction perpendicular to the drawing sheet of FIG. 12. The light source 113 also extends in a direction perpendicular to the drawing sheet of FIG. 12, emits the linear light toward the original supported on the support member 114, and is moved with the full-speed scan mirror 111 to scan the original. The copying machine is disclosed in Japanese Patent Application laid open for inspection under Publication No. 63(1988)-36277.
When the copying machine starts an image scanning operation, the full-speed scan mirror 111 takes a position indicated at solid lines. During the scanning operation, the mirror 111 is moved in a direction indicated at arrow. When the mirror 111 reaches a position indicated at broken lines, the scanning operation is ended. Meanwhile, the half-speed scan mirror 112 is provided by a pair of mirrors and, for the scanning operation, the mirror 112 is moved in a direction indicated at arrow from a position indicated at solid lines to a position indicated at broken lines. Since during the scanning operation the ratio, 2 (=2/1), of the speed of the full-speed scan mirror 111 to that of the half-speed scan mirror 112 is maintained, the original image is detected or read with accuracy by the line sensor 115.
The copying machine additionally has an image-recording optical system including a light source (not shown), such as a semiconductor laser, which emits a light beam which is incident to a rotary polygon mirror 117 as a light deflector. After the light beam is reflected and deflected by the polygon mirror 117 which is being rotated at a uniform angular speed, the light beam passes through an image-forming lens 118 such as an f.theta. lens and is reflected by a reflecting mirror 119, so that the reflected light scans a photoconductive drum 120 at a uniform speed. Thus, an electrostatic latent image corresponding to image data used to drive the laser as the light source is formed in an outer circumferential surface of the drum 120.
FIG. 13 shows another image-reading optical system or scanner which is employed in, e.g., a conventional copying machine. The optical scanner includes a semiconductor laser 201 as a light source, a rotary polygon mirror 202 which deflects the light emitted by the light source 201, an image-forming lens 203 which converges the light deflected by that polygon mirror 202, so the the converged light scans an original 206 having an original image thereon, a single photodiode 261 which detects the light reflected from the original 206, and a control device 207 which controls the light source 201 and the polygon mirror 202. More specifically, the control device 207 controls the light source 201 to emit a laser beam which is incident to the polygon mirror 202 which is being rotated at a uniform speed. The laser beam deflected by the rotary polygon mirror 202 is converged by the image-forming lens 203, so that the converged light scans the original 206 by irradiating each of different positions on the original 206 at a corresponding one of different times. The light reflected from the original 206 is detected by the photodiode 261. The photodiode 261 produces an electric signal indicative of a detected intensity of the light reflected from the original 206, and supplies the electric signal to the control device 207. Based on the electric signal supplied from the photodiode 261, the control device 207 produces and stores, in a memory, image data indicative of the original image carried on the original 206. An example of this optical scanner is disclosed in Japanese Patent Application laid open for inspection under Publication No. 1(1989)-155370.
A light intensity detected by the photodiode 261 when the light converged by the image-forming lens 203 irradiates a position or picture element where an image is present, is significantly different from a light intensity detected by the same 261 when the light irradiates a position where no image is present. Thus, the image data produced by the control device 207 comprise data indicative of the light intensities detected by the photodiode 261 when the light irradiates each of the positions or picture elements on the original 206.
However, in the first prior image-reading optical system shown in FIG. 12, the array of CCDs 115, i.e., line sensor is employed to detect an original image. Since each of the CCDs 115 detects the intensity of light reflected from a single position or picture element on the original, a greater number of CCDs are needed to increase the degree of resolution of the detected image. However, the number of light sensing elements (e.g., CCDs) of a line sensor cannot exceed a certain upper limit. In addition, an increased number of light sensing elements cost higher.
In the second prior image-reading optical system shown in FIG. 13, the single photodiode 261 as the light detecting device is employed to read an original image and provide image data indicative of the read image. However, since an original has not only positions or picture elements near to the photodiode 261 but also positions remote from the same 261, the photodiode 261 cannot produce electric signals with a uniform magnitude for all the positions on the original, if the original has no image thereon. Thus, the accuracy of image reading of the optical system is low.